Evidence for Topological Edge States in a Large Energy Gap near the Step Edges on the Surface of ZrTe5

Abstract

Two-dimensional topological insulators with a large bulk band gap are promising for experimental studies of quantum spin Hall effect and for spintronic device applications. Despite considerable theoretical efforts in predicting large-gap two-dimensional topological insulator candidates, none of them have been experimentally demonstrated to have a full gap, which is crucial for quantum spin Hall effect. Here, by combining scanning tunneling microscopy/spectroscopy and angle-resolved photoemission spectroscopy, we reveal that ZrTe5 crystal hosts a large full gap of ∼100meV on the surface and a nearly constant density of states within the entire gap at the monolayer step edge. These features are well reproduced by our first-principles calculations, which point to the topologically nontrivial nature of the edge states.

Alternate abstract:

Plain Language Summary

Two-dimensional topological insulators conduct electrons on their surfaces and act as insulators in their bulk. In the edge states, electrons propagating in opposite directions have opposite spins, leading to a quantum spin Hall effect. To maximize the quantum spin Hall effect, the bulk band gap must be large enough so that thermally excited carriers in the bulk are sufficiently few to contribute to the electron and spin transport. However, such two-dimensional topological insulators with a large fully opened band gap have not been experimentally demonstrated, and the quantum spin Hall effect has only been demonstrated at ultralow temperatures in previous experiments. Here, we show that ZrTe5 crystals exhibit a large energy gap with topological edge states above 4 K.

Using scanning tunneling microscopy/spectroscopy and angle-resolved photoemission spectroscopy of single ZrTe5 crystals, we observe a large full band gap of approximately 0.1 eV on the surface of the ZrTe5 crystal and a finite density of states, which is nearly constant within the entire gap, at the monolayer step edge. We show that the conductance attains a maximum at the edge of the sample and that the edge states decay exponentially into the bulk. These observations are well reproduced by our first-principles calculations, which point to the topologically nontrivial nature of the edge states.

Such a large energy gap with topological edge states observed in ZrTe5 is promising for the quantum spin Hall devices and ideal (i.e., dissipationless) wires operating at relatively high temperatures. We expect that our findings will stimulate applications of topological insulators and propel “topotronics” as the next generation of microelectronics and spintronics.